Hyperphosphatemia frequently accompanies diseases associated with inadequate renal function, hypoparathyroidism, and certain other medical conditions. Hyperphosphatemia is typically defined as possessing a serum phosphate level of over about 6 mg/dL. The condition, especially if present over extended periods of time, leads to severe abnormalities in calcium and phosphorus metabolism and can be manifested by aberrant calcification in joints, lungs, and eyes.
Therapeutic efforts to reduce serum phosphate include dialysis, reduction in dietary phosphate, and oral administration of insoluble phosphate binders to reduce gastrointestinal absorption. Dialysis and reduced dietary phosphate are generally unsuccessful in adequately reversing hyperphosphatemia. Further difficulties in these therapeutic regimens include the invasive nature of dialysis and the difficulties in modifying dietary habits in the latter therapy.
The oral administration of certain phosphate binders has also been suggested. Phosphate binders include calcium or aluminum salts. Calcium salts have been widely used to bind intestinal phosphate and prevent absorption. The ingested calcium combines with phosphate to form insoluble calcium phosphate salts such as Ca3(PO4)2, CaHPO4, or Ca(H2PO4)2. Different types of calcium salts, including calcium carbonate, acetate (such as PhosLo® calcium acetate tablets), citrate, alginate, and ketoacid salts have been utilized for phosphate binding. This class of therapeutics generally results in hypercalcemia due to absorption of high amounts of ingested calcium. Hypercalcemia has been indicated in many serious side effects, such as cardiac arrhythmias, renal failure, and skin and visceral calcification. Frequent monitoring of serum calcium levels is required during therapy with calcium-based phosphate binders.
Aluminum-based phosphate binders, such as Amphojel® aluminum hydroxide gel, have also been used for treating hyperphosphatemia. These compounds complex with intestinal phosphate to form highly insoluble aluminum phosphate; the bound phosphate is unavailable for absorption by the patient. Prolonged use of aluminum gels leads to accumulations of aluminum, and often to aluminum toxicity, accompanied by such symptoms as encephalopathy, osteomalacia, and myopathy.
Selected ion exchange resins have also been suggested for use in binding phosphate. Those tested include Dowex® anion-exchange resins in the chloride form, such as XF 43311, XY 40013, XF 43254, XY 40011, and XY 40012. These resins have several drawbacks for treatment of hyperphosphatemia, including poor binding efficiency, necessitating use of high dosages for significant reduction of absorbed phosphate.
Certain anion exchange polymers, such as sevelamer hydrochloride (as disclosed in U.S. Pat. No. 5,667,775), have shown effectiveness as a phosphate sequestrant capable of lowering elevated serum phosphate levels. Sevelamer hydrochloride includes a polymer having pendent groups therefrom, the pendent groups having a single amino group. It would be desirable to develop new polymers with similar or more favorable phosphate binding properties.